Recently, the performance and functionality of digital cameras and digital movie cameras that use some solid-state imaging device such as a CCD and a CMOS (which will be sometimes referred to herein as an “image sensor”) have been enhanced to an astonishing degree. In particular, the size of a pixel structure for use in a solid-state imaging device has been further reduced these days thanks to development of semiconductor device processing technologies, thus getting an even greater number of pixels and drivers integrated together in a solid-state imaging device. As a result, the resolution of an image sensor has lately increased significantly from one million pixels to ten million pixels in a matter of few years. Meanwhile, the greater the number of pixels in an image sensor, the lower the intensity of the light falling on a single pixel (which will be referred to herein as a “light intensity”) and the lower the sensitivity of camera tends to be.
On top of that, in a normal color camera, a subtractive color filter that uses a pigment as a dye is arranged over each photosensing section of an image sensor, and therefore, the optical efficiency achieved is rather low. In a Bayer color filter, which uses a combination of one red (R) pixel, two green (G) pixels and one blue (B) pixel as a fundamental unit, the R filter transmits an R ray but absorbs G and B rays, the G filter transmits a G ray but absorbs R and B rays, and the B filter transmits a B ray but absorbs R and G rays. That is to say, each color filter transmits only one of the three colors of R, G and B and absorbs the other two colors. Consequently, the light ray used by each color filter is only approximately one third of the visible radiation falling on that color filter.
To overcome such a problem of decreased sensitivity, Patent Document No. 1 discloses a technique for increasing the intensity of the light received by attaching an array of micro lenses to a photodetector section of an image sensor. According to this technique, the incoming light is condensed with those micro lenses, thereby substantially increasing the aperture ratio. And this technique is now used in almost all solid-state imaging devices. It is true that the aperture ratio can be increased substantially by this technique but the decrease in optical efficiency by color filters still persists.
Thus, to avoid the decrease in optical efficiency and the decrease in sensitivity at the same time, Patent Document No. 2 discloses a solid-state imaging device that has a structure for taking in as much incoming light as possible by using dichroic mirrors and micro lenses in combination. Such a device uses a combination of dichroic mirrors, each of which does not absorb light but selectively transmits only a component of light falling within a particular wavelength range and reflects the rest of the light falling within the other wavelength ranges. Each dichroic mirror selects only a required component of the light, makes it incident on its associated photosensing section and transmits the rest of the light. FIG. 10 is a cross-sectional view of such an image sensor as the one disclosed in Patent Document No. 2.
In the image sensor shown in FIG. 10, the light that has reached a condensing micro lens 11 has its luminous flux adjusted by an inner lens 12, and then impinges on a first dichroic mirror 13, which transmits a red (R) ray but reflects rays of the other colors. The light ray that has been transmitted through the first dichroic mirror 13 is then incident on a photosensitive cell 23 that is located right under the first dichroic mirror 13. On the other hand, the light ray that has been reflected from the first dichroic mirror 13 impinges on a second dichroic mirror 14 adjacent to the first dichroic mirror 13. The second dichroic mirror 14 reflects a green (G) ray and transmits a blue (B) ray. The green ray that has been reflected from the second dichroic mirror 14 is incident on a photosensitive cell 24 that is located right under the second dichroic mirror 14. On the other hand, the blue ray that has been transmitted through the second dichroic mirror 14 is reflected from a third dichroic mirror 15 and then incident on a photosensitive cell 25 that is located right under the dichroic mirror 15. In this manner, in the image sensor shown in FIG. 10, the visible radiation that has reached the condensing micro lens 11 is not absorbed into color filters but its RGB components can be detected by the three photosensitive cells non-wastefully.
Likewise, Patent Documents Nos. 3 and 4 also disclose a technique for increasing the optical efficiency by taking advantage of reflection and transmission of incoming light. FIG. 11(a) illustrates a dichroic prism for a two tube color camera as disclosed in Patent Document No. 3. On the other hand, FIG. 11(b) illustrates a dichroic prism for a three tube color camera as disclosed in Patent Document No. 4. The dichroic prism 21 shown in FIG. 11(a) splits the incoming light into a G ray and the other rays (i.e., R and B rays), while the dichroic prism 22 shown in FIG. 11(b) splits the incoming light into R, G and B rays.
As described above, the image sensors disclosed in Patent Documents Nos. 2, 3 and 4 do not use any color filter that selectively transmits a light ray falling within a particular wavelength range and absorbs light rays falling within the other wavelength ranges. Instead, all of those image sensors use an optical element for selectively transmitting and reflecting light rays falling within particular wavelength ranges. That is to say, by using such color separating optical elements, the incoming light can be used much more efficiently.
Each of those image sensors disclosed in Patent Documents Nos. 2, 3 and 4, however, should have as many photosensitive cells as the optical elements to use or the colors to separate. That is why to sense red, green and blue rays, for example, the number of photosensitive cells provided should be tripled compared to a situation where conventional color filters are used.
Thus, to overcome such problems with the prior art, Patent Document No. 5 discloses a technique for increasing the optical efficiency by using dichroic mirrors and reflected light, although some loss of the incoming light is involved. FIG. 12 is a partial cross-sectional view of an image sensor that adopts such a technique. As shown in FIG. 12, dichroic mirrors 32 and 33 are embedded in a light-transmitting resin 31. Specifically, the dichroic mirror 32 transmits a G ray and reflects R and B rays, while the dichroic mirror 33 transmits an R ray and reflects G and B rays.
Such a structure cannot receive a B ray at its photosensing section but can sense R and G rays entirely under the following principle. First, if an R ray impinges on the dichroic mirrors 32 and 33, the R ray is reflected from the dichroic mirror 32, is totally reflected from the interface between the light-transmitting resin 31 and the air, and then strikes the dichroic mirror 33. Then, almost all of the R ray that has impinged on the dichroic mirror 33 will be incident on the photosensing section by way of the organic dye filter 35 and the micro lens 36 that transmit the R ray, even though only a part of the light is reflected from the metal layer 37. On the other hand, if a G ray impinges on the dichroic mirrors 32 and 33, the G ray is reflected from the dichroic mirror 33, is totally reflected from the interface between the light-transmitting resin 31 and the air, and then strikes the dichroic mirror 32. Then, almost all of the G ray that has impinged on the dichroic mirror 32 will eventually be incident on the photosensing section with virtually no loss by way of the organic dye filter 34 and the micro lens 36 that transmit the G ray.
According to the technique disclosed in Patent Document No. 5, only one of the three colors of RGB is lost but light rays of the other two colors can be received with almost no loss based on the principle described above. That is why there is no need to provide photosensing sections for all of the three colors of RGB. In this case, comparing such an image sensor to an image sensor that uses only organic dye filters, it can be seen that the sensitivity can be doubled by this technique because the image sensor that uses organic dye filters will achieve an optical efficiently of just one-third but this technique will achieve an optical efficiency of two-thirds. Nevertheless, even if such a technique is adopted, the optical efficiency cannot be 100%, as one out of the three colors should be sacrificed.
Citation List
Patent Literature
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